The present invention concerns a method of sterilizing a packaging material by means of a sterilizing agent in liquid form.
Aseptic packaging technology has for a long time been used for packaging foodstuffs and the like, especially products sensitive to bacteria and storage, in order to give the product an extended life so that it can be kept with retained fresh qualities for longer periods of time from the day of packaging without risk of it being spoiled or deteriorating. The technology which is well known to the specialist in the area can for example start out from the fact that the product and the packaging material are each subjected to a sterilizing treatment for the purpose of neutralizing harmful micro-organisms occurring in the product or the packaging material, in particular pathogenic bacteria, and that the treated product is thereafter enclosed in the sterilized packaging material under sterile conditions in order to avoid a bacterial reinfection of the sterilized product.
Aseptic packagings for milk, juice and similar liquid foods are now most frequently produced with the aid of modern, rational packaging machines of the type which, from a strip or sheet of plastic coated paper or cardboard material, forms, fills and closes packagings under aseptic conditions. From, for example, a strip a known packaging machine produces aseptic packagings for milk through the strip being taken for the purpose of serialization through a bath containing 10-35% by weight of hydrogen peroxide within a chamber essentially completely screened from the environment (what is known as an aseptic house). After the passage through the bath the strip is taken through the pinching between press rollers in order to remove the surplus sterilizing agent from the strip and take it back to the bath, after which the strip, without coming into contact with the machine environment is taken into the forming and filling chamber of the machine which is likewise essentially completely screened from the environment. The strip is shaped into a tube through the two longitudinal edges of the strip being joined to each other in a longitudinal overlap joint at the same time as the packaging material in the tube formed is heated by means of sterile hot air in order to vaporize and drive away the residue of accompanying sterilizing agent from the packaging material. The tube is filled with the appropriate previously sterilised contents, heat treated milk, which is fed to the tube through a filler pipe opening into the tube, and separated into closed, filled packagings through repeated transverse sealings of the tube across the longitudinal axis of the tube. The packagings are separated from each other through cuts in the transverse seals and are subsequently given the desired geometric final shape, usually of parallelpiped type, before outfeed of the finished aseptic packagings from the machine. During the whole process an overpressure of sterile hot air is maintained in the shaping and filling chamber in order to prevent unsterile environmental air penetrating and reinfecting the sterilized contents and the packaging material.
A precondition for achieving good serialization of the packaging material in the above described known method is that the whole strip, after passing through the sterilizing bath, is covered by a coherent film of sterilizing agent in order to ensure that all parts of the packaging material are effectively sterilized. The film should in addition preferably be thin and of even thickness in order to facilitate and make more effective the subsequent driving away of the stabilizing agent in the shaping and filling chamber. These two conditions have been shown to be difficult to fulfill in practice and it not infrequently happens that the sterilizing agent exhibits an irregular film distribution over the strip surface with alternating thicker and thinner film zones, which not only leads to an uneven, unpredictable sterilization effect but also makes the drying process more difficult during heating. It sometimes also happens that the sterilizing agent is completely missing along certain areas, while other areas of the strip show island-like concentrations of the sterilizing agent, which thus further worsens the possibility of achieving the intended even serialization effect over the whole strip. Another disadvantage with the above described and other known methods which employ a bath of sterilizing agent through which the strip is taken is what is called the edge suction phenomenon which entails that the exposed fibrous material in the longitudinal cut edges easily absorb the sterilizing agent in liquid form which is sucked into and retained in the fibrous layer of the strip and finally accompanies the packaging material into the finished packaging. Since the inward facing cut edges in the finished packaging are always well protected no risk occurs that the accompanying sterilizing agent should come in contact with and affect the contents of the packaging, but on the contrary the risk is great that the liquid absorbed will at least locally cause deterioration in the rigidity and stability of form of the packaging at the same time as it of course entails an unnecessary loss of sterilizing agent.
The aim of the present invention is therefore to give indications about a new way of sterilizing a packaging material by means of a sterilizing agent in liquid form without subsequent problems of the type described above.
This aim is achieved according to the invention through the fact that a method of the type described in the introduction is given the characteristics that the packaging material is first discharged in order to eliminate electrostatic surface charges occurring on the strip, that the sterilizing fluid is subsequently applied to the packaging material in a finely distributed, electrostatically charged form to create a coherent, homogeneous film, and that the packaging material is finally heated to drive away the sterilizing fluid from the ready sterilised packaging material.
Through first eliminating the surface charges on the packaging material and thereafter applying the sterilizing agent form in a finely distributed, electrostatically charged form it has been shown that the finely distributed sterilizing agent is not only easily received on the packaging material but in addition it easily coalesces without hindrance from inhibiting electrostatic repulsion forces and forms a homogeneous film completely covering the packaging material with the same good predictable sterilizing effect. A further advantage which is gained through the process according to the invention is that the film applied can be made very thin, but of even thickness, which considerably facilitates the subsequent heating of the strip to drive away the sterilizing agent. Furthermore the problem with edge suction is completely avoided since the cut edges of the strip do not need to come in contact with the sterilizing agent.
Further advantageous and practical embodiments of the method according to the invention have further been given the characteristics mentioned in the sub-claims below.